The tapping or diversion of a portion of the electromagnetic energy in a main or primary transmission line is a well-known problem. For example, with cable television systems, the main transmission line runs along a street. Each subscriber along the street who desires cable TV has a branch line coming into the subscriber's house which is tapped off of the main transmission line. (In the cable television (CATV) industries, the main type of transmission line used is coaxial cable.)
Directional coupler taps accept a signal coming from the source port or head end and pass the signal through to a load port while diverting part of the signal to a branch line. The directional coupler discriminates against signals which travel in the opposite direction; that is, signals entering the load port are prevented from being passed through to the branch port, and signals entering the branch port are prevented from being passed through to the load port.
A drawback of previous directional coupler taps is that they operate over a relatively narrow frequency range. This problem is especially acute at very high frequencies. Many directional couplers include transformers in their design. The major factor limiting the upper frequency limit of couplers now in use is the difficulty of maintaining pure inductive coupling at high frequencies.
Another drawback of existing directional couplers is that their directivity is relatively poor at higher frequencies. (This is especially critical for applications involving emerging communications systems.) Directivity is a measure of the degree of discrimination of the received power in the branch port when equal power is applied at the source and load ports. There is an increasing need to provide two-way communications to the average home. The characteristics of the directional coupler--including its directivity--determine how the bi-directional signals are handled in such systems. Accordingly, a directional coupler with a high directivity over a broad bandwidth is required.
A representative directional coupler using inductive coupling is disclosed in U.S. Pat. No. 3,048,798, issued Aug. 7, 1962. This coupler, while effective for its originally-intended purposes, does not provide the extremely wide frequency pass band desirable for certain present-day applications. The '798 coupler teaches the use of two cross-connected transformers which provide the necessary inductive coupling.
The '798 coupler works efficiently over the frequency range in which the transformer has nearly pure inductive coupling. However, as is typical of directional couplers which employ transformers, the '798 coupler has extremely poor performance above approximately 700 megahertz.